Green’s Dictionary of Slang

1. a gratuity; a bribe.

1830		Egan Finish to the Adventures of Tom and Jerry (1889) 189: You must tip. It is the sort of sweetener we folks expect, to make everything right!
1847		[A. Harris] (con. 1820s) Settlers & Convicts 89: The handsome ‘sweeteners’ (bribes) which old D—’s profits enabled him to give to the constables .
1851		‘Epistle from Joe Muggins’s Dog’ in Era (London) 21 Sept. 5/1: Playman Morris, ov ‘Hungerford’ Market [...] he’s had a sweetner or two.
1869		Western Dly Press 17 Apr. 3/5: ‘Sweeteners’ to government Officials [...] The first thing said to him by the contractor was ‘I suppose, sir, you expect the usual 2½ per cent commission?’ This meant a preliminary present, or ‘sweetener’ of £5,000.
1880		Manchester Eve. News 10 Jan. 2/3: Diplomacy, which in this instance assumed the shape of a money bribe and an even more effectual ‘sweetener’ in the shape of duty free rum and gin.
1930		(con. 1910–20s) D. Mackenzie Hell’s Kitchen 120: Sweetener ... a gratuity; bribe; also refers to coaxing.
1948		Dundee Courier 16 Aug. 2/2: Now it seems that the Argentinian cattle-breeders got only two-thirds of their £10,000,000 sweetener from their Government.
1955		B. Hill Boss of Britain’s Underworld 3: A thief had to pay many sorts of people [...] bribes and sweeteners.
1978		H.R. Haldeman Ends of Power 163: As a further sweetener, the President offered him his choice of an overseas trip on ‘special assignment’.
1989		J. Morton Lowspeak.
2011		J.J. Connolly Viva La Madness 367: I’d made a point of transferring the sweetener [...] to an account I held in the Jamaican branch of a Canadian bank.

2. a scolding, a ‘talking-to’.

1846		Bell’s Life in Sydney 28 Nov. 3/3: After a ‘sweetener’ from the Captain, Kitty was discharged and her man of business bound over the keep the peace.
1851		Life in Boston & N.Y. (Boston, MA) 4 Jan. n.p.: [U]nless he reforms his conduct, we shall shortly give hima sweetener.
1864		A.F. Hill Our Boys 145: The colonel [...] with an oath commonly called a ‘sweetener,’ roared out peremptorily: ‘Who the h-ll fired that gun?’.

3. (US) a drink.

1848

Spirit of the Times (N.Y.) 18 Mar. in A.P. Hudson Humor of the Old Deep South (1936) 43: Let’s take a sweetener and go to bed.

4. a hard blow.

1848		Bell’s Life in Sydney 18 Mar. 1/4: Terry [...] popped a sweetener on the ivories.
1857		Bell’s Life in Victoria (Melbourne) 23 May 4/1: [B]efore Alec could deliver a ‘sweetener’ the opportunity of doing so [...] was gone.
1868		H.L. Williams Night in the New Hotel in Darkey Drama 4 43: I’ll gib you one ob my sweeteners if you don’t chain up! (flourishes umbrella).

5. an encouragement.

1865

Leaves from Diary of Celebrated Burglar 147/2: This was indeed anything but a ‘sweetener.’ The bare idea of such a loss prevented us sleeping at night.

6. (UK Und.) in pl., the lips.

1890–1904

Farmer & Henley Sl. and Its Analogues.

7. the penis.

1904

Lustful Memoirs of a Young and Passionated Girl 24: When he saw by her countenance that she was beginning to feel some pleasure he began slowly to move his sweetener out and in.